Hindlimb Muscle Blood Flow Research Articles

Redistribution of blood flow during exercise is impaired in heart failure with preserved ejection fraction

Exercise intolerance is a cardinal symptom in heart failure with preserved ejection fraction (HFpEF). The underlying pathophysiological mechanisms of HFpEF and their influence on exercise intolerance remains poorly understood. This study investigated the role of re-distribution of blood flow in modulating the increase in hindlimb muscle blood flow during HFpEF.We proposed that exercise intolerance is partially contributed by an impaired ability to divert blood flow from peripheral organs such as the kidney during exercise. We utilised an ovine model of HFpEF in which hypertension was established in aged female sheep. We then examined the mean arterial pressure, heart rate, cardiac output, renal blood flow and hindlimb blood flow response to treadmill exercise in both groups of conscious animals.Animals with HFpEF had elevated left ventricular end diastolic pressures (13 ± 6 vs. 0 ± 1 mmHg, p<0.05) as well as enlarged cardiomyocytes compared to normotensive controls. Exercise in the control animals led to increases in cardiac output and heart rate. These responses were significantly attenuated in HFpEF (increase of 4.5 ± 1 vs. 8 ± 1 L/min, p<0.05) and heart rate (increase of 32 ± 3 vs. 41 ± 4 bpm, p<0.05). In a separate group of instrumented animals, the increase in blood flow to the hindlimb vasculature was attenuated in the HFpEF animals compared to control animals (increase of 2 ± 1 vs. 4 ± 2 L/min, p<0.05). There was a reduction in renal blood flow in the control animals (-150 ± 50 ml/min) and this change was reversed to an increase in the HFpEF animals (+50 ± 20 ml/min).Our results suggest that the exercise intolerance observed in our ovine model of HFpEF may in part be due to an impaired ability to regulate blood flow to the kidney and hindlimb muscles during exercise. RR gratefully acknowledges granting support from the Health Research Council of New Zealand, the University of Auckland Faculty Research Development Fund and the National Heart Foundation of New Zealand. This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Effects of comorbid type II diabetes mellitus and heart failure on rat hindlimb and respiratory muscle blood flow during treadmill exercise.

In rats with type II diabetes mellitus (T2DM) compared with nondiabetic healthy controls, muscle blood flow (Q̇m) to primarily glycolytic hindlimb muscles and the diaphragm muscle are elevated during submaximal treadmill running consequent to lower skeletal muscle mass, a finding that held even when muscle mass was normalized to body mass. In rats with heart failure with reduced ejection fraction (HF-rEF) compared with healthy controls, hindlimb Q̇m was lower, whereas diaphragm Q̇m is elevated during submaximal treadmill running. Importantly, T2DM is the most common comorbidity present in patients with HF-rEF, but the effect of concurrent T2DM and HF-rEF on limb and respiratory Q̇m during exercise is unknown. We hypothesized that during treadmill running (20 m·min-1; 10% incline), hindlimb and diaphragm Q̇m would be higher in T2DM Goto-Kakizaki rats with HF-rEF (i.e., HF-rEF + T2DM) compared with nondiabetic Wistar rats with HF-rEF. Ejection fractions were not different between groups (HF-rEF: 30 ± 5; HF-rEF + T2DM: 28 ± 8%; P = 0.617), whereas blood glucose was higher in HF-rEF + T2DM (209 ± 150 mg/dL) compared with HF-rEF rats (113 ± 28 mg/dL; P = 0.040). Hindlimb muscle mass normalized to body mass was lower in rats with HF-rEF + T2DM (36.3 ± 1.6 mg/g) than in nondiabetic HF-rEF counterparts (40.3 ± 2.7 mg/g; P < 0.001). During exercise, Q̇m was elevated in rats with HF-rEF + T2DM compared with nondiabetic counterparts to the hindlimb (HF-rEF: 100 ± 28; HF-rEF + T2DM: 139 ± 23 mL·min-1·100 g-1; P < 0.001) and diaphragm (HF-rEF: 177 ± 66; HF-rEF + T2DM: 215 ± 93 mL·min-1·100g-1; P = 0.035). These data suggest that the pathophysiological consequences of T2DM on hindlimb and diaphragm Q̇m during treadmill running in the GK rat persist even in the presence of HF-rEF.NEW & NOTEWORTHY Herein, we demonstrate that rats comorbid with heart failure with reduced ejection fraction (HF-rEF) and type II diabetes mellitus (T2DM) have a higher hindlimb and respiratory muscle blood flow during submaximal treadmill running (20 m·min-1; 10% incline) compared with nondiabetic HF-rEF counterparts. These data may carry important clinical implications for roughly half of all patients with HF-rEF who present with T2DM.

Impact Of Cell-free Hemoglobin On Exercising Muscle Vascular Control In Rats

Hemolysis associated with Sickle Cell Disease (SCD) compromises nitric oxide (NO) bioavailability and results in a plethora of cardiopulmonary and skeletal muscle complications causing severe exercise intolerance. Recent evidence suggests that cell-free Hb reduces NO bioavailability and lowers the skeletal muscle microvascular PO2 during contractions, likely due to a reduction in blood flow. Despite these observations, the effects of Hb on skeletal muscle vascular control during locomotory exercise remain unknown. PURPOSE: We tested the hypothesis that acute exposure to Hb would increase mean arterial pressure (MAP) and decrease hindlimb muscle blood flow in the exercising rat. METHODS: MAP and hindlimb skeletal muscle blood flow (fluorescent microspheres) were measured in male Sprague-Dawley rats (3-6 months, n=8) during submaximal treadmill running (20 ml/min, 5% grade) following a vehicle (0.2 ml of saline) and Hb (50 mg/kg) infusion. RESULTS: Relative to control, Hb resulted in a significantly greater exercising MAP (control: 137 ± 3, Hb: 150 ± 3 mmHg, p<0.05) and blood [lactate] (control: 2.51 ± 0.25, Hb: 3.13 ± 0.42 mM, p<0.05). Total exercising hindlimb skeletal muscle blood flow (control: 179 ± 14, Hb: 111 ± 7, ml/min/100 g, p<0.05) and vascular conductance (control: 1.34 ± 0.13, Hb: 0.75 ± 0.05 ml/min/100 g/mmHg, p<0.05) were lower following Hb infusion when compared to control. CONCLUSION: These data support the hypothesis that free Hb impairs vascular control and lowers skeletal muscle O2 delivery during exercise and provides a potential mechanism by which hemolytic diseases like SCD impair exercise tolerance in humans. Support: NIH-P30DK048520 (SKF), NIH-1R01HL125642-01A1 (DCI)

Effect of chronic heart failure in older rats on respiratory muscle and hindlimb blood flow during submaximal exercise

Submaximal exercise diaphragm blood flow (BF) is elevated in young chronic heart failure (CHF) rats, while it is unknown if this occurs in older animals. Respiratory and hindlimb muscle BFs (radiolabeled microspheres) were measured at rest and during submaximal exercise (20m/min, 5% grade) in older healthy (n=7) and CHF (n=6) Fischer 344X Brown Norway rats (27–29 mo old). Older CHF, compared to healthy, rats had greater (p<0.01) left ventricular end-diastolic pressure and right ventricle and lung weight (normalized to body weight). During submaximal exercise, respiratory and hindlimb muscle BFs increased (p<0.02) in both groups, while diaphragm BF was higher (CHF: 257±32; healthy: 121±9mL/min/100g, p<0.01) and hindlimb BF lower (CHF: 111±10; healthy: 133±12mL/min/100g, p=0.04) in older CHF compared to healthy rats. Submaximal exercise hindlimb BF was negatively related (r=−0.93; p=0.03) to diaphragm BF in older CHF rats. During submaximal exercise, diaphragm BF is elevated in older CHF compared to healthy rats in proportion to the compromised hindlimb BF.

Acute inhibition of ATP-sensitive K+ channels impairs skeletal muscle vascular control in rats during treadmill exercise.

The ATP-sensitive K(+) (KATP) channel is part of a class of inward rectifier K(+) channels that can link local O2 availability to vasomotor tone across exercise-induced metabolic transients. The present investigation tested the hypothesis that if KATP channels are crucial to exercise hyperemia, then inhibition via glibenclamide (GLI) would lower hindlimb skeletal muscle blood flow (BF) and vascular conductance during treadmill exercise. In 27 adult male Sprague-Dawley rats, mean arterial pressure, blood lactate concentration, and hindlimb muscle BF (radiolabeled microspheres) were determined at rest (n = 6) and during exercise (n = 6-8, 20, 40, and 60 m/min, 5% incline, i.e., ~60-100% maximal O2 uptake) under control and GLI conditions (5 mg/kg intra-arterial). At rest and during exercise, mean arterial pressure was higher (rest: 17 ± 3%, 20 m/min: 5 ± 1%, 40 m/min: 5 ± 2%, and 60 m/min: 5 ± 1%, P < 0.05) with GLI. Hindlimb muscle BF (20 m/min: 16 ± 7%, 40 m/min: 30 ± 9%, and 60 m/min: 20 ± 8%) and vascular conductance (20 m/min: 20 ± 7%, 40 m/min: 33 ± 8%, and 60 m/min: 24 ± 8%) were lower with GLI during exercise at 20, 40, and 60 m/min, respectively (P < 0.05 for all) but not at rest. Within locomotory muscles, there was a greater fractional reduction present in muscles comprised predominantly of type I and type IIa fibers at all exercise speeds (P < 0.05). Additionally, blood lactate concentration was 106 ± 29% and 44 ± 15% higher during exercise with GLI at 20 and 40 m/min, respectively (P < 0.05). That KATP channel inhibition reduces hindlimb muscle BF during exercise in rats supports the obligatory contribution of KATP channels in large muscle mass exercise-induced hyperemia.

B-24 Free Communication/Poster - Assessments with Exercise

Smooth muscle cell hyperpolarization contributes to exercise induced skeletal muscle hyperemia mediated, in part, by K+ efflux. The ATP-gated potassium (KATP) channel is a class of inwardly rectifying K+ channels which may link cellular metabolic status to vasomotor tone across the prodigious metabolic transients seen with exercise. PURPOSE: The aim of this investigation was to test the hypothesis that blockade of KATP channels via glibenclamide (GLI) would lower hindlimb skeletal muscle blood flow (BF) and vascular conductance (VC) during submaximal, large muscle mass exercise in healthy rats. METHODS: In 14 adult male Sprague Dawley rats mean arterial pressure (MAP), blood [lactate], and hindlimb muscle BF (radiolabelled microspheres, 57Co and 85Sr) were determined either at rest (n = 6) or during exercise (n = 8; 5% incline, 20 m·min-1) under both control (CON) and GLI conditions (5 mg·kg-1, i.a). RESULTS: Both at rest and during exercise, MAP was higher (Rest, CON: 130 ± 6, GLI: 152 ± 8; Exercise, CON: 140 ± 4, GLI: 147 ± 4 mmHg, p < 0.05) and heart rate (HR) was lower (Rest, CON: 440 ± 16, GLI: 410 ± 18; Exercise, CON: 559 ± 4, GLI: 538 ± 10 bpm, p < 0.05) with GLI compared to CON. Hindlimb muscle BF and VC were lower with GLI during exercise but not at rest. Specifically, GLI decreased BF in 12, and VC in 16, of the 28 individual hindlimb muscles or muscle parts during exercise (p < 0.05). Additionally, blood [lactate] (CON: 2.0 ± 0.3; GLI: 4.1 ± 0.9 mmol·L-1, p < 0.05) was higher during exercise with GLI. CONCLUSION: These data indicate that KATP channel blockade with GLI attenuates the skeletal muscle BF response to submaximal exercise in healthy animals. Our results suggest that KATP channels are obligatory for large muscle mass, exercise induced hyperemia.FIGURE 1

Dose dependent effects of nitrate supplementation on cardiovascular control and microvascular oxygenation dynamics in healthy rats

High dose nitrate (NO3−) supplementation via beetroot juice (BR, 1mmol/kg/day) lowers mean arterial blood pressure (MAP) and improves skeletal muscle blood flow and O2 delivery/utilization matching thereby raising microvascular O2 pressure (PO2mv). We tested the hypothesis that a low dose of NO3− supplementation, consistent with a diet containing NO3− rich vegetables (BRLD, 0.3mmol/kg/day), would be sufficient to cause these effects. Male Sprague–Dawley rats were administered a low dose of NO3− (0.3mmol/kg/day; n=12), a high dose (1mmol/kg/day; BRHD, n=6) or tap water (control, n=10) for 5days. MAP, heart rate (HR), blood flow (radiolabeled microspheres) and vascular conductance (VC) were measured during submaximal treadmill exercise (20m/min, 5% grade, equivalent to ∼60% of maximal O2 uptake). Subsequently, PO2mv (phosphorescence quenching) was measured at rest and during 180s of electrically-induced twitch contractions (1Hz, ∼6V) of the surgically-exposed spinotrapezius muscle. BRLD and BRHD lowered resting (control: 139±4, BRLD: 124±5, BRHD: 128±9mmHg, P<0.05, BRLD vs. control) and exercising (control: 138±3, BRLD: 126±4, BRHD: 125±5mmHg, P<0.05) MAP to a similar extent. For BRLD this effect occurred in the absence of altered exercising hindlimb muscle(s) blood flow or spinotrapezius PO2mv (rest and across the transient response at the onset of contractions, all P>0.05), each of which increased significantly for the BRHD condition (all P<0.05). Whereas BRHD slowed the PO2mv kinetics significantly (i.e., >mean response time, MRT; control: 16.6±2.1, BRHD: 23.3±4.7s) following the onset of contractions compared to control, in the BRLD group this effect did not reach statistical significance (BRLD: 20.9±1.9s, P=0.14). These data demonstrate that while low dose NO3− supplementation lowers MAP during exercise it does so in the absence of augmented muscle blood flow, VC and PO2mv; all of which are elevated at a higher dose. Thus, in healthy animals, a high dose of NO3− supplementation seems necessary to elicit significant changes in exercising skeletal muscle O2 delivery/utilization.

The effects of dietary fish oil on exercising skeletal muscle vascular and metabolic control in chronic heart failure rats

Impaired vasomotor control in chronic heart failure (CHF) is due partly to decrements in nitric oxide synthase (NOS) mediated vasodilation. Exercising muscle blood flow (BF) is augmented with polyunsaturated fatty acid (PUFA) supplementation via fish oil (FO) in healthy rats. We hypothesized that FO would augment exercising muscle BF in CHF rats via increased NO-bioavailability. Myocardial infarction (coronary artery ligation) induced CHF in Sprague-Dawley rats which were subsequently randomized to dietary FO (20% docosahexaenoic acid, 30% eicosapentaenoic acid, n = 15) or safflower oil (SO, 5%, n = 10) for 6-8 weeks. Mean arterial pressure (MAP), blood [lactate], and hindlimb muscles BF (radiolabeled microspheres) were determined at rest, during treadmill exercise (20 m·min(-1), 5% incline) and exercise + N(G)-nitro-l-arginine-methyl-ester (l-NAME) (a nonspecific NOS inhibitor). FO did not change left ventricular end-diastolic pressure (SO: 14 ± 2; FO: 11 ± 1 mm Hg, p > 0.05). During exercise, MAP (SO: 128 ± 3; FO: 132 ± 3 mm Hg) and blood [lactate] (SO: 3.8 ± 0.4; FO: 4.6 ± 0.5 mmol·L(-1)) were not different (p > 0.05). Exercising hindlimb muscle BF was lower in FO than SO (SO: 120 ± 11; FO: 93 ± 4 mL·min(-1)·100 g(-1), p < 0.05) but was not differentially affected by l-NAME. Specifically, 17 of 28 individual muscle BF's were lower (p < 0.05) in FO demonstrating that PUFA supplementation with FO in CHF rats does not augment muscle BF during exercise but may lower metabolic cost.

Impact of dietary nitrate supplementation via beetroot juice on exercising muscle vascular control in rats

Dietary nitrate (NO(3)(-)) supplementation, via its reduction to nitrite (NO(2)(-)) and subsequent conversion to nitric oxide (NO) and other reactive nitrogen intermediates, reduces blood pressure and the O(2) cost of submaximal exercise in humans. Despite these observations, the effects of dietary NO(3)(-) supplementation on skeletal muscle vascular control during locomotory exercise remain unknown. We tested the hypotheses that dietary NO(3)(-) supplementation via beetroot juice (BR) would reduce mean arterial pressure (MAP) and increase hindlimb muscle blood flow in the exercising rat. Male Sprague-Dawley rats (3-6 months) were administered either NO(3)(-) (via beetroot juice; 1 mmol kg(-1) day(-1), BR n = 8) or untreated (control, n = 11) tap water for 5 days. MAP and hindlimb skeletal muscle blood flow and vascular conductance (radiolabelled microsphere infusions) were measured during submaximal treadmill running (20 m min(-1), 5% grade). BR resulted in significantly lower exercising MAP (control: 137 ± 3, BR: 127 ± 4 mmHg, P < 0.05) and blood [lactate] (control: 2.6 ± 0.3, BR: 1.9 ± 0.2 mm, P < 0.05) compared to control. Total exercising hindlimb skeletal muscle blood flow (control: 108 ± 8, BR: 150 ± 11 ml min(-1) (100 g)(-1), P < 0.05) and vascular conductance (control: 0.78 ± 0.05, BR: 1.16 ± 0.10 ml min(-1) (100 g)(-1) mmHg(-1), P < 0.05) were greater in rats that received BR compared to control. The relative differences in blood flow and vascular conductance for the 28 individual hindlimb muscles and muscle parts correlated positively with their percentage type IIb + d/x muscle fibres (blood flow: r = 0.74, vascular conductance: r = 0.71, P < 0.01 for both). These data support the hypothesis that NO(3)(-) supplementation improves vascular control and elevates skeletal muscle O(2) delivery during exercise predominantly in fast-twitch type II muscles, and provide a potential mechanism by which NO(3)(-) supplementation improves metabolic control.

Acute ascorbic acid and hindlimb skeletal muscle blood flow distribution in old rats: rest and exercise

Excess reactive oxygen species are implicated in the impaired peripheral vascular function evident during exercise in older individuals. We tested the hypothesis that an acute infusion of the antioxidant ascorbic acid (AA) in old rats would improve antioxidant capacity and reduce oxidative stress and, therefore, elevate hindlimb muscle blood flow at rest and during treadmill exercise in muscles containing principally type I and IIa muscle fibers. Total and individual hindlimb skeletal muscle blood flow was measured (radiolabeled microspheres) in old rats (26-28 months) at rest (n = 8) and during treadmill exercise (n = 8; 20 m·min⁻¹, 5% grade) before and after AA treatment (76 mg·(kg body mass)⁻¹ intra-arterial (i.a.) injection). AA elevated total antioxidant capacity (rest, ~37%; and exercise, 31%) and reduced oxidative stress (~26%, exercise only). AA reduced resting total hindlimb muscle blood flow (control, 25 ± 3; AA, 16 ± 2 mL·min⁻¹·(100 g)⁻¹; p < 0.05) and blood flow to 8 of 28 individual muscles with no fiber-type correlation (p > 0.05). During exercise there was no effect of AA on total hindlimb muscle blood flow (control, 154 ± 14; AA, 162 ± 13 mL·min⁻¹·(100 g)⁻¹; p > 0.05) or blood flow to any individual muscle. This disconnect between whole-body antioxidant status and skeletal muscle blood flow in old rats mandates consideration when pursuing antioxidant treatments experimentally or clinically in older populations.

The power–duration relationship of high‐intensity exercise: from mathematical parameters to physiological mechanisms

The power–duration relationship of high‐intensity exercise: from mathematical parameters to physiological mechanisms

Critical speed in the rat: implications for hindlimb muscle blood flow distribution and fibre recruitment

Critical speed (CS) constitutes an important metabolic and performance demarcator. However, active skeletal muscle blood flow distribution specifically surrounding CS remains unknown. We tested the hypotheses that CS could be accurately determined in the running rat and that measurement of hindlimb inter- and intramuscular blood flow below and above CS would support that the greatest muscle fibre recruitment above, relative to below, CS occurs in the predominantly glycolytic muscles. Seven male Sprague-Dawley rats performed five constant-speed tests to exhaustion at speeds between 95 and 115% of the speed that elicited to determine CS. Subsequent constant-speed tests were performed at speeds incrementally surrounding CS to determine time to exhaustion, V(O2), and hindlimb muscle blood flow distribution. Speed and time to exhaustion conformed to a hyperbolic relationship (r(2) = 0.92 ± 0.03) which corresponded to a linear 1/time function (r(2) = 0.93 ± 0.02) with a CS of 48.6 ± 1.0 m min(-1). Time to exhaustion below CS was ∼ 5× greater (P < 0.01) than that above. Below CS V(O2) stabilized at a submaximal value (58.5 ± 2.5 ml kg(-1) min(-1)) whereas above CS (81.7 ± 2.5 ml kg(-1) min(-1)) increased to (84.0 ± 1.8 ml kg(-1) min(-1), P > 0.05 vs. above CS). The 11 individual muscles or muscle parts that evidenced the greatest blood flow increases above, relative to below, CS were composed of ≥ 69% Type IIb/d/x muscle fibres. Moreover, there was a significant correlation (P < 0.05, r = 0.42) between the increased blood flow above expressed relative to below CS and the percentage Type IIb/d/x fibres found in the individual muscles or muscle parts. These data validate the powerful CS construct in the rat and identify that running above CS, relative to below CS, incurs disproportionate blood flow increases (indicative of recruitment) in predominantly highly glycolytic muscle fibres.

The Effects of Ascorbic Acid Supplementation on Muscle Blood Flow in Aged Rats

During exercise aged individuals exhibit endothelial dysfunction and decreased levels of whole-limb blood flow, both of which may be linked mechanistically to age-related increases in reactive oxygen species (ROS). Ascorbic acid (AA) reduces levels of ROS and has been shown to alleviate vascular and hyperemic dysfunction at rest and during small muscle mass exercise in humans (Kirby et al. J Physiol., 2009). However, the effect of AA on vascular function and blood flow (BF) to individual muscles during dynamic whole-body exercise is not known. PURPOSE: To test the hypothesis that a single high-dose infusion of AA would increase BF to the hindlimb musculature of old rats at rest and during treadmill running. METHODS: 18 old (∼28 months) Fischer 344 × Brown Norway rats were randomized into rest (n=9) and exercise (n=9) groups. BF to the hindlimb (28 individual muscles and muscle parts) was evaluated via radiolabeled microspheres before and after intra-arterial AA administration (76 mg/kg in 3 ml heparinized saline, 30 minute infusion) at rest and during submaximal treadmill running (20 m/min, 5% grade). Total antioxidant capacity (TAC) and thiobarbituric acid reactive species (TBARS) were measured before and after AA to determine the ability of this dose of AA to increase levels of plasma antioxidants and decrease levels of ROS, respectively. RESULTS: At rest: AA increased TAC (∼37%, P < 0.05) but did not change TBARS (Pre: 6.8 ± 0.7 vs. Post: 7.0 ± 1.0 μM, P > 0.05). AA decreased total hindlimb BF (Pre: 25 ± 3 vs Post: 16 ± 2 ml/min/100g, P < 0.05) and BF to 8 of the 28 individual muscles that were evaluated. During exercise: TAC was increased (∼35%, P < 0.05) and TBARS were decreased (Pre: 9.8 ± 2.0 vs Post: 6.1 ± 0.5 μM, P < 0.05). However, there was no effect on either total hindlimb BF (Pre: 154 ± 14 vs. Post: 162 ± 13, P > 0.05) or BF to any of the individual muscles that were evaluated. CONCLUSIONS: Increased TAC via AA infusion reduces hindlimb muscle BF at rest but not during whole-body dynamic exercise. Thus, although this specific AA dose decreased TBARS, there was no evidence that AA supplementation increases blood flow to the locomotor muscles during whole-body exercise. Whether other doses of AA or alternative antioxidants provide beneficial outcomes in other models remains unknown. Support: AHA Grant 070090Z

Effects of neuronal nitric oxide synthase inhibition on resting and exercising hindlimb muscle blood flow in the rat

Nitric oxide (NO) derived from endothelial NO synthase (eNOS) is an integral mediator of vascular control during muscle contractions. However, it is not known whether neuronal NOS (nNOS)-derived NO regulates tissue hyperaemia in healthy subjects, particularly during exercise. We tested the hypothesis that selective nNOS inhibition would reduce blood flow and vascular conductance (VC) in rat hindlimb locomotor muscle(s), kidneys and splanchnic organs at rest and during dynamic treadmill exercise (20 m min(-1), 10% grade). Nineteen male Sprague-Dawley rats (555 +/- 23 g) were assigned to either rest (n = 9) or exercise (n = 10) groups. Blood flow and VC were determined via radiolabelled microspheres before and after the intra-arterial administration of the selective nNOS inhibitor S-methyl-L-thiocitrulline (SMTC, 2.1 +/- 0.1 micromol kg(-1)). Total hindlimb muscle blood flow (control: 20 +/- 2 ml min(-1) 100g(-1), SMTC: 12 +/- 2 ml min(-1) 100g(-1), P < 0.05) and VC (control: 0.16 +/- 0.02 ml min(-1) 100 g(-1) mmHg(1), SMTC: 0.09 +/- 0.01 ml min(-1) 100 g(-1) mmHg(-1), P < 0.05) were reduced substantially at rest. Moreover, the magnitude of the absolute reduction in blood flow and VC correlated (P < 0.05) with the proportion of oxidative muscle fibres found in the individual muscles or muscle parts of the hindlimb. During exercise, total hindlimb blood flow (control: 108 +/- 7 ml min(-1) 100 g(-1), SMTC: 105 +/- 8 ml min(-1) 100 g(-1)) and VC (control: 0.77 +/- 0.06 ml min(-1) 100g(-1) mmHg(-1); SMTC: 0.70 +/- 0.05 ml min(-1) 100g(-1) mmHg(-1)) were not different (P > 0.05) between control and SMTC conditions. SMTC reduced (P < 0.05) blood flow and VC at rest and during exercise in the kidneys, adrenals and liver. These results enhance our understanding of the role of NO-mediated circulatory control by demonstrating that nNOS does not appear to subserve an obligatory role in the exercising muscle hyperaemic response in the rat.

Nitric oxide synthase inhibition during treadmill exercise reveals fiber-type specific vascular control in the rat hindlimb

The control of vascular tone during exercise is highly complex and integrated. Specifically, in regards to the contribution of nitric oxide (NO), the observed magnitude and muscle fiber-type dependency of the NO contribution to exercise hyperemia may differ depending on the timing of NO synthase (NOS) inhibition with respect to the exercise bout (i.e., administration prior to vs. during exercise). We tested the hypothesis that, in the presence of prior cyclooxygenase inhibition (indomethacin, 5 mg/kg(-1)), NOS inhibition (N(G)-nitro-L-arginine methyl ester, L-NAME; 10 mg/kg) administered during submaximal treadmill exercise would blunt blood flow and vascular conductance (VC) in the hindlimb muscle(s) of the rat with the greatest reductions in blood flow and VC occurring in the predominantly oxidative muscles. Adult female Wistar rats (n = 10, age: 3-4 mo) ran on a motor-driven treadmill (20 m/min, 10% grade). Total and regional hindlimb muscle blood flow and VC were determined via radiolabeled microspheres before (control) and after L-NAME administration during exercise. L-NAME reduced (P < 0.05) total hindlimb muscle blood flow (control: 123 + or - 10, L-NAME: 103 + or - 7 ml x min(-1) x 100g(-1)) and VC (control: 0.95 + or - 0.09, L-NAME: 0.63 + or - 0.05 ml x min(-1) x 100g(-1) x mmHg(-1)). There was a significant correlation (r = 0.51, P < 0.05) between the absolute reductions in VC after L-NAME and the percent sum of type I and IIa fibers in the individual muscles and muscle parts; however, there was no correlation (P = 0.62) when expressed as blood flow. Surprisingly, the highly oxidative muscles demonstrated a marked ability to maintain oxygen delivery, which differs substantially from previous reports of L-NAME infusion prior to exercise in these muscles. The demonstration that NO is an important regulator of blood flow and VC in the rat hindlimb during treadmill exercise, but that the fiber-type dependency of NO is altered markedly when NOS inhibition is performed during, vs. prior to, exercise, lends important insights into the integrated nature of vascular control during exercise.

Assessment by Thallium-201 of Hindlimb Muscle Blood Flow in Rats during Recovery after Hindlimb Unloading

[Purpose] We aimed to evaluate the muscle blood perfusion by the uptake of the radioisotope thallium-201 (201Tl) in 5 hindlimb muscles, the soleus (Sol), plantaris (Pla), gastrocnemius (Gas), extensor digitorum longus (EDL), and tibialis anterior (TA). [Subjects] The subjects were 36 male Wistar rats. [Methods] 201Tl uptake rates were calculated by spectrometry and visualized by autoradiography (ARG) in hindlimb unloading (U) and reloading (R) rats. [Results] The Sol 201Tl uptake rates were significantly lower in U groups (p<0.01) than in the control group. Meanwhile the reloading day-4 group had higher values in Sol than the control (p<0.01). In U groups, the EDL and TA uptake rates were higher than in the other 3 muscles (Sol, Pla, and Gas). [Conclusion] We concluded that during 21 days of hindlimb unloading and subsequent recovery by reloading, blood capillary perfusion assessed by 201Tl in hindlimb muscles changes in rats. The results confirm the utility of 201Tl in quantitative and qualitative evaluation of skeletal muscle activity and blood perfusion, which has applications in rehabilitation and sports science.

Interaction of Femoral Artery Ligation and TVaining on Microvascular PO2 and Blood Flow in Rat Soleus Muscle

The microvascular PO2 (PmO2) reflects the dynamic balance between oxygen delivery and oxygen uptake, and is the driving force for oxygen flux into muscle cells. Femoral artery ligation reduces hindlimb muscle blood flow in rats which is partially restored with the development of collateral circulation, and is enhanced with exercise training (Am J Physiol 287: H2434, 2004). It is not known how effectively this collateral circulation restores oxygen delivery. PURPOSE: To test the hypothesis that increased collateral flow would restore PO2m and that this would be enhanced with exercise training. METHODS: The left femoral artery of female Sprague-Dawley rats (∼275 g) was either left intact (sham) or double-ligated. Sham and ligated rats were divided into four experimental groups; sedentary sham (SED-SH; n=10), sedentary ligated (SED-LIG; n=12), exercise trained sham (ET-SH; n=8), and exercise trained ligated (ET-LIG; n=10). Daily treadmill training began 48 hrs after surgery. The exercise intensity and duration were increased progressively to 17 m/min (at 20 degrees) for up to 60 min/day, as tolerated. Experiments were performed 21–28 days later. PmO2 (phosphorescence quenching) and blood flow (radiolabeled microspheres) were measured in the left soleus muscle at rest and during 1 Hz contractions. RESULTS: Soleus PmO2 in the SED-LIG rats was lower than in the SED-SH rats (33 ± 3 vs. 19 ± 3 mmHg). Training did not alter resting PmO2 (ET-SH, 33 ± 2; ET-LIG, 19 ± 2 mmHg). During contractions, PmO2 was lower in the SED-LIG (19 ± 3 mmHg) compared to SED-SH (28 ± 3 mmHg). In contrast, PmO2 during contractions in the trained groups was higher than the sedentary groups (ET-SH, 35 ± 4; ET-LIG, 26 ± 3 mmHg). Soleus blood flow during contractions was less in the SED-LIG rats (27 ±13 vs. 64 ± 14 ml/min/100 g). Soleus flow was doubled in ET-SH rats, but was unchanged in ET-LIG rats (140 ± 18 and 34 ± 15 ml/min/100 g). CONCLUSIONS: Even after chronic femoral artery ligation, there were marked deficits in blood flow and PmO2 in the soleus muscle. Low intensity training increased PmO2 during contractions in the femoral artery ligated rats. This was not associated with an increase in blood flow, and suggests there was a redistribution of flow to support oxygen delivery to the working muscle.

Transneuronal tracing of neural pathways that regulate hindlimb muscle blood flow

Despite considerable interest in the neural mechanisms that regulate muscle blood flow, the descending pathways that control sympathetic outflow to skeletal muscles are not adequately understood. The present study mapped these pathways through the transneuronal transport of two recombinant strains of pseudorabies virus (PRV) injected into the gastrocnemius muscles in the left and right hindlimbs of rats: PRV-152 and PRV-BaBlu. To prevent PRV from being transmitted to the brain stem via motor circuitry, a spinal transection was performed just below the L2 level. Infected neurons were observed bilaterally in all of the areas of the brain that have previously been shown to contribute to regulating sympathetic outflow: the medullary raphe nuclei, rostral ventrolateral medulla (RVLM), rostral ventromedial medulla, A5 adrenergic cell group region, locus coeruleus, nucleus subcoeruleus, and the paraventricular nucleus of the hypothalamus. The RVLM, the brain stem region typically considered to play the largest role in regulating muscle blood flow, contained neurons infected following the shortest postinoculation survival times. Approximately half of the infected RVLM neurons were immunopositive for tyrosine hydroxylase, indicating that they were catecholaminergic. Many (47%) of the RVLM neurons were dually infected by the recombinants of PRV injected into the left and right hindlimb, suggesting that the central nervous system has a limited capacity to independently regulate blood flow to left and right hindlimb muscles.

Intrauterine growth restriction induces increased capillary density and accelerated type I fiber maturation in newborn pig skeletal muscles

Humans with low birth weight exhibit evidences of vascular dysfunction. Recent findings indicate a microvascular rarefaction in skeletal muscles soon after postnatal development in rats suffered by intrauterine protein restriction. To examine the effects of intrauterine growth restriction on capillary density, muscle fiber distribution and accompanying muscular and systemic circulation immediately after birth, studies were conducted on 1-day-old anesthetized normal weight (n = 7) and intrauterine growth restricted (n = 6) piglets. Cardiac output and hind limb muscle blood flow were measured by colored microspheres. Counting of type I fibers and skeletal capillary numbers was done by immunohistochemical staining. Increased proportion of type I fibers and capillary density was found in the flexor digitalis superficialis and gastrocnemius medialis (P < 0.05) in newborn IUGR piglets. Furthermore, a marked correlation was shown between capillary density and type I fiber fraction for all flexor muscles studied (P < 0.05). Moreover, cardiac output and muscular blood flow were markedly increased in IUGR piglets (P < 0.05). Correspondingly, total peripheral resistance, as well as vascular resistance, of hind limb flexors appeared significantly decreased (P < 0.05). Compromised intrauterine environmental conditions leading to fetal growth restriction provokes coordinated structural and functional adaptation of skeletal muscles.

Rat hindlimb muscle blood flow during level and downhill locomotion.

During eccentrically biased exercise (e.g., downhill locomotion), whole body oxygen consumption and blood lactate concentrations are lower than during level locomotion. These general systemic measurements indicate that muscle metabolism is lower during downhill exercise. This study was designed to test the hypothesis that hindlimb muscle blood flow is correspondingly lower during downhill vs. level exercise. Muscle blood flow (determined by using radioactive microspheres) was measured in rats after 15 min of treadmill exercise at 15 m/min on the level (L, 0 degrees) or downhill (D, -17 degrees). Blood flow to ankle extensor muscles was either lower (e.g., white gastrocnemius muscle: D, 9 +/- 2; L, 15 +/- 1 ml. min-1. 100 g-1) or not different (e.g., soleus muscle: D, 250 +/- 35; L, 230 +/- 21 ml. min-1. 100 g-1) in downhill vs. level exercise. In contrast, blood flow to ankle flexor muscles was higher (e.g., extensor digitorum longus muscle: D, 53 +/- 5; L, 31 +/- 6 ml. min-1. 100 g-1) during downhill vs. level exercise. When individual extensor and flexor muscle flows were summed, total flow to the leg was lower during downhill exercise (D, 3.24 +/- 0.08; L, 3.47 +/- 0. 05 ml/min). These data indicate that muscle blood flow and metabolism are lower during eccentrically biased exercise but are not uniformly reduced in all active muscles; i.e., flows are equivalent in several ankle extensor muscles and higher in ankle flexor muscles.